The geology history in Mill Creek Park

By Jim Thornton

Ohio certified volunteer naturalist

There are many geological wonders in the world, and all of these wonders have interesting histories. Did you know that the rock beneath your feet has its own interesting story? You can get up close and personal with a stroll through Mill Creek Park.

If you go to the Mahoning River where Mill Creek ends, the rock layer exposed below the first dam on Mill Creek is Mississippian-period sandstone. Sandstone is a sedimentary rock formed from tiny pieces of other dissolved rocks.

The Earth’s surface is made up of large sections or plates that are in motion. They have bumped into each other, formed mountain ranges, and have moved great distances. The North American plate has done its share of traveling and bumping. The Mississippian layer was formed some 319 to 340 million years ago near the equator. When this rock layer was formed, the area we live in was a shallow sea. Years of deposits on the sea floor formed this layer and the fossils found there like brachiopods are evidence of its history under the sea.

The next layer of sandstone above the dam is Pennsylvanian-period sandstone. When this layer was formed, the still-tropical North American plate had bumped into the African plate, lifting our area out of sea and forming very tall mountains to our east. Over time, around 300 millions years ago, these mountains eroded, sending sand downhill into our area, which became this layer of sandstone. The Pennsylvanian sandstone filled in the irregularly eroded Mississippian rock, giving us various layers together with other deposits of shales, coals and fossils (Some such as Sigillaria, look like logs in the rocks).

As you move upstream from the river and leave the Narrows past Lake Glacier Dam, you will notice the Mill Creek Valley becomes much wider. This pattern of narrow gorge-like area and wide valley repeats itself all the way to Lake Newport. This is caused by glaciers and flowing water. The last glacier, the Wisconsin was a sheet of ice about a half-mile thick. Glaciers moved great amounts of rocks and soils along with them. Before the Wisconsin glacier there was a previous Mill Creek that fed a previous version of the Mahoning River. When the current Mill Creek follows the course of the old Mill Creek, it is cutting though the small rocks and soils. It clears a wide area. However, when it is cutting its own way through the sandstone, it cuts a much more narrow passage, showing as the Narrows and the Gorge.

Mill Creek Park has many other stories to tell, so be sure to look around. Remember, you may have trouble with geology in the beginning, but it is just fine when you get off to a rocky start. To learn some basics, check out

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